The present invention relates to mechanisms for transferring motion into an isolated region while maintaining the isolation thereof (i.e., isolated transfer), and more particularly to mechanisms that transfer rotary motion into such a region.
In many applications it is desirable to rotate an object located in an isolated region without disturbing the isolated region (e.g., without significantly impacting the isolated region's temperature, pressure, atmospheric content, etc.). For instance, in the semiconductor industry it is often necessary to rotate a wafer located within a vacuum or other processing chamber before, during, and/or after wafer processing. Such rotation may be performed manually (e.g., by an operator of the processing equipment) or automatically (e.g., with a stepper motor or similar apparatus). Absent a mechanism for isolated transfer of motion into an isolated region (e.g., the vacuum chamber), a motor for rotating the wafer must be contained in the, vacuum chamber, or the vacuum chamber must be vented, exposed to the surrounding atmosphere, and the wafer manually rotated. Both approaches are unacceptable in many cases as both expose the wafer to contaminants (either from the motor or from the atmosphere surrounding the vacuum chamber), and the later significantly increases wafer processing time.
In addition to the contamination concerns of the semiconductor industry, the semiconductor industry and many other industries (e.g., robotics, lamp manufacturing, aerospace/defense, fiber optic manufacturing, chemical/hydrocarbon processing, and the like), often require rotary motion within an isolated region containing toxic or otherwise environmentally hazardous gases/chemicals, high or low temperatures, or other similar conditions that mandate isolation.
For all the above situations, and in many others, a mechanism for isolated transfer of rotary motion into an isolated region (i.e., a rotary feedthrough) is required. Typically, rotary feedthroughs include a first stage isolation region which is coupled to the isolated region (e.g., to the process chamber). A first rotatable shaft is located outside the first stage isolation region and operatively couples to a second rotatable shaft located within the first stage isolation region. The second shaft extends from the first stage isolation region into the isolation region. The first and second rotatable shafts are coupled so that rotation of the first shaft outside the first stage isolation region rotates the second rotatable shaft inside the first stage isolation region (and thus rotates the portion of the second shaft which extends into the isolated region). Some mechanism is provided for coupling the two shafts in an isolated manner (i.e., such that no fluid passageway exists between the first stage isolation region and a region external thereto).
One conventional rotary feedthrough configuration employs magnetic coupling as shown in Gilmore U.S. Pat. No. 5,113,102. Specifically, an air tight housing is attached to the isolated region (e.g., a vacuum chamber) so that the housing and the isolated region are in fluid communication. The housing contains permanent magnets rigidly attached to a shaft which extends from the housing into the isolated region. Instead of employing a first shaft outside the isolated region and a second shaft inside the isolated region, only one shaft is employed which extends from the housing into the isolated region. To rotate the shaft within the isolated region a stator winding surrounds the exterior surface of the housing. The stator winding generates a magnetic field that causes the permanent magnets within the housing to rotate, which in turn rotates the shaft within the isolated region. The stator winding and permanent magnets thus form a brushless electric motor, and rotation of the shaft within the isolated region is controllable from outside the isolated region.
Because magnetic materials are temperature sensitive (e.g., subject to a phase transition at the Curie temperature which renders the magnetic materials non-magnetic or significantly less magnetic), most magnetically coupled rotary feedthroughs cannot operate above about 220.degree. C., making such feedthroughs impractical for many semiconductor processing applications. Furthermore, magnetic coupling is not sufficiently strong to support the high torque levels required of many rotary feedthroughs. Magnetic materials are also very sensitive to radiation and to hydrogen and, for reliable rotary feedthrough operation, must be protected from environments containing either radioactive materials or hydrogen. Accordingly magnetically coupled rotary feedthroughs are not suitable for high temperature, high torque, radioactive or certain chemical environments.
An alternative rotary feedthrough configuration that employs mechanical rather than magnetic coupling is shown in Balter U.S. Pat. No. 4,683,763. FIG. 1 is a side elevational view of such a conventional mechanically coupled rotary feedthrough (represented as conventional rotary feedthrough 11). The conventional rotary feedthrough 11 comprises a collar 13 and a housing 15 which surrounds the collar 13. The housing 15 has a first housing end 15a through which a first rotary shaft 17 rotatably passes (i.e., such that the first shaft may rotate with respect to the first housing end 15a) and rotatably couples to the collar 13, and a second housing end 15b through which a second rotary shaft 19 rotatably passes and rotatably couples to the collar 13. The first rotary shaft 17 has an enlarged slanted end 21 and is rotatably coupled to the first housing end 15a via first housing end bearings 23 and to the collar 13 via enlarged slanted end bearings 25. The second rotary shaft 19 has a reduced slanted portion 27 and is rotatably coupled to the second housing end 15b via second housing end bearing 29 and to the collar 13 via reduced slanted portion bearings 31.
A bellows 33 is coupled between the second housing end 15b and the collar 13 to form a first stage isolation region 35. A second stage isolation region such as a vacuum chamber 37 can be coupled to the conventional rotary feedthrough 11 as shown in FIG. 1. Note the region within the bellows 33 is referred to herein as a first stage isolation region because this region is isolated from the rest of the housing and because a fluid path may or may not exist along the second rotary shaft 19 between the first stage isolation region 35 and the vacuum chamber 37.
To connect the conventional rotary feedthrough 11 to the vacuum chamber 37, the housing 15 is sealingly attached (e.g., forming an air tight seal) to the vacuum chamber 37 via bolt holes 39a, 39b. The second rotary shaft 19 extends into the vacuum chamber 37.
In operation, the first rotary shaft 17 is rotated manually or via a motor, causing the collar 13 (which is coupled to the enlarged slanted end 21 of the first rotary shaft 17) to move in a circular arcing motion. The enlarged slanted end bearings 25 which couple the collar 13 to the first rotary shaft 17 isolate the collar 13 from a portion of the rotary force exerted by the first rotary shaft 17. Nonetheless, the circular arcing motion of the collar 13 causes the second rotary shaft 19, which is coupled to the collar 13, to rotate. Rotary motion is thereby transferred from the first rotary shaft 17 to the second rotary shaft 19 in an isolated manner.
Because the enlarged slanted end bearings 25 and the reduced slanted portion bearings 31 are imperfect, the rotary motion of the first and second rotary shafts 17, 19 exerts a rotary force on the collar 13. In response to this rotary force the collar 13 attempts to rotate. However, because the bellows 33 is rigidly attached to both the collar 13 and the second end plate 15b, rotation of the collar 13 would twist the bellows 33 and thus would interfere with the bellow's operation. Accordingly, the bellows 33 may detach from either the collar 13 or the second end plate 15b, or otherwise fail. Thus, in order to prevent rotation of the collar 13 the bellows 33 of U.S. Pat. No. 4,683,763 presumably must be fabricated from a semi-rigid material (such as a metal) that can withstand the rotary force applied to the bellows due to rotation of the first and secondary rotary shafts 17, 19.
Accordingly, while conventional mechanically coupled rotary feedthroughs are less sensitive to elevated temperatures than are magnetically coupled feedthroughs, conventional mechanically coupled rotary feedthroughs are still unsuitable for high torque applications. That is, because the bearings used in such feedthroughs are imperfect, when a high torque is exerted on the first rotary shaft 17, a significant torque may be transferred to the bellows 33 via the collar 13, straining the bellows 33 and subjecting it to wear during normal use.
Accordingly, conventional mechanically coupled rotary feed-through designs require the use of more expensive, semi-rigid bellows in order to prevent the collar 13 from rotating. The semi-rigid bellows thus are exposed to repeated strain which in turn shortens the bellow's life. Additionally, such conventional, metal, semi-rigid bellows are incompatible with many chemical environments (e.g., hydrofluoric acid, etc.).
Thus a need exists for an improved and cost-effective method and apparatus for transmitting rotary motion into an isolated region under normal, as well as high torque, high temperature or chemically reactive conditions.